librarify langfilter too

langfilter/lib/Lang.hs

@@ -0,0 +1,31 @@
+module Lang where
+
+import Text.Pandoc.Definition
+import Data.Char (toLower)
+import qualified Data.Text as Text
+import System.IO
+import Data.Text (Text)
+
+data Lang = Lántas deriving (Eq, Show)
+
+type Vars = (?lang :: Maybe Lang, ?defColor :: Text)
+
+toText :: Maybe MetaValue -> IO (Maybe Text)
+toText (Just (MetaInlines [Str s])) = toText (Just (MetaString s)) -- ugh
+toText (Just (MetaString s)) = pure $ Just s
+toText Nothing = pure Nothing
+toText (Just ℓ) = do
+  hPutStrLn stderr $ "[WARN] expected a string, got: " <> show ℓ
+  pure Nothing
+
+toLang :: Maybe MetaValue -> IO (Maybe Lang)
+toLang m = do
+  mres <- fmap (Text.map toLower) <$> toText m
+  case mres of
+    Just res -> do
+      if res `elem` ["laantas", "lántas"] then
+        pure $ Just Lántas
+      else do
+        hPutStrLn stderr $ "[WARN] unknown language: " <> show res
+        pure Nothing
+    Nothing -> pure Nothing